The present invention relates to reconfigurable microwave lenses and shutters. In particular, the present invention relates to reconfigurable microwave lenses and shutters using cascaded frequency selective surfaces and polyimide-macro-electro-mechanical systems.
Antennas are used to radiate and receive radio frequency signals. The transmission and reception of radio frequency signals is useful in a broad range of activities. For instance, radio wave communication systems are desirable where communications are transmitted over large distances. In addition, the transmission and reception of radio wave signals is useful in connection with obtaining position information regarding distant objects.
Antennas are generally formed to receive and transmit signals having frequencies within defined ranges. In addition to such frequency selectivity, antennas having a beam that can be pointed or steered in space can be provided. The pointing of an antenna beam can be accomplished by physically moving the radiator element or elements of the antenna. The beam of an antenna can also be steered electronically. The steering of an antenna beam is useful because it allows an antenna to focus on a distant receiver or transmitter, maximizing the gain of the antenna with respect to the distant transmitter or receiver. In addition, the pointing of an antenna beam allows the location of distant objects to be determined with respect to the antenna. Furthermore, by moving (or scanning) a beam of radio frequency radiation, a wide area can be surveyed by a single antenna.
In order to control the frequencies received by or emitted from an antenna, frequency selective surfaces (FSS) are known. With reference now to FIG. 1A, a band pass FSS 100 in accordance with the prior art is illustrated. In the band pass FSS of FIG. 1A, resonant slots 104 are formed in a layer of metal 108 overlaying a substrate 112. The slots behave in the same fashion as a resonant L-C shunt admittance pair, as illustrated in FIG. 1B, for which the resonant frequency occurs when       ω    2    =            1              L        ⁢                  xe2x80x83                ⁢        C              .  
The admittance, Yp of the L-C shunt admittance pair may be defined as a function of frequency as Yp=jB=j       (                  ω        ⁢                  xe2x80x83                ⁢        C            -              1                  ω          ⁢                      xe2x80x83                    ⁢          L                      )    .
By altering the width and length of the slots, and/or their relationship to one another, the effective values of L and C may be changed, thereby changing the resonant frequency response of the band pass FSS. Such band pass FSS structures can be designed to have very low transmission losses within the pass band. However, a conventional band pass FSS 100 such as the one illustrated in FIG. 1 cannot be controlled to selectively alter its transmission pass band, and associated transmission phase, while the FSS 100 is operatively connected to an antenna. Therefore, a conventional band pass FSS 100 is not able to selectively modify an antenna beam, or, specifically, to scan the beam towards a target.
Microwave lenses that allow an antenna beam to be scanned by modifying the refractive index of a panel made from an artificial dielectric are known. For example, in a RADANT(copyright) lens an artificial dielectric is formed from grids of cut wires and continuous wires, with diodes bridging the gap between cut wire segments. By biasing the diodes either on or off the index of refraction can be changed, thereby altering the phase of transmitted radio frequency radiation. However, such devices require the integration of thousands of discrete, lossy components (e.g., diodes). In addition, RADANT(copyright) lenses are heavy, and therefore are difficult to deploy, particularly in mobile or in space-based applications.
Phased array antennas that provide scanning beams are also known. In a phased array antenna, the phase of the radio frequency signals provided to individual antenna radiator elements is altered across the surface of the antenna. Conventional phased array antennas typically require the use of a large number of semiconductor switches or micro-electro mechanical (MEMs) devices to control the phase of the individual radiator elements. Accordingly, conventional phased array antennas are complicated and expensive to implement. In addition, the use of lossy components such as semiconductor switches and traditional micro-electro-mechanical devices results in large insertion losses.
Radio frequency shutters that can be selectively opened or closed to transmit or reflect radio frequency signals are also known. For example, an electronic diode shutter may be constructed by connecting diodes across the midpoint of slot elements in a conducting FSS sheet. By biasing the diodes either on or off, the resonant characteristics of the slots can be changed, thereby detuning the slots and altering the transmission and reflection properties of the FSS. Such shutters may be used to control the radar cross section of antennas or to protect antenna receiver circuitry from being damaged by high-power incident radio frequency signals while in the off state. However, shutter implementations employing thousands of discrete components entail the same types of liabilities as do diode lenses. Namely, complexity, loss, operating power, and weight.
For the above stated reasons, it would be desirable to provide a lens for use in connection with radio frequency antennas that allowed the phase of a transmitted radio frequency wave to be controlled, while exhibiting low insertion losses. Furthermore, it would be advantageous to provide such a device to permit the scanning or pointing of radio frequency radiation that required low power to operate and was relatively simple to construct and implement. In addition, it would be desirable to provide such a lens that was reliable in operation and that was suitable for use in connection with a wide variety of applications. It would also be desirable to provide shutter capability to the aforementioned lens, or to any antenna, for use in control of antenna radar cross section and/or protection from antenna damage caused by incident high-power radio frequency signals.
In accordance with the present invention, a frequency selective surface (FSS) that can be electrically detuned to provide insertion phase and amplitude control of radio frequency radiation propagating through the structure is provided. In general, the present invention uses frequency selective surfaces that are locally detuned in order to control the localized admittance, and hence localized insertion phase, of each surface. Further, a method for implementing such localized de-tuning, and hence localized insertion phase control, is described wherein two or three tightly coupled frequency selective surfaces are separated from one another by a small distance that can be electro-mechanically altered. By cascading a sufficient number of individually controllable tightly coupled groups of such surfaces, a full 360 degree change in insertion phase can be produced through the aggregate of surfaces, which is sufficient to scan the beam of a fixed beam antenna that transmits or receives through them. The same detuning technique when applied globally to an FSS can be used to increase or decrease the transmission amplitude of the FSS, thereby producing the effect of a shutter within a fixed frequency band.
In accordance with an embodiment of the present invention, an electromechanically reconfigurable microwave lens is provided that uses frequency selective surfaces in conjunction with polyimide macro-electromechanical systems (PMEMS). The following embodiment describes a two-layer implementation. According to such an embodiment, a first FSS sheet comprising a first array of unit cells formed on a first surface is provided. A second FSS sheet comprising a second array of unit cells is formed on a second surface, positioned so that the first and second arrays occupy parallel planes and at least partially overlap. In accordance with an embodiment of the present invention, the unit cells consist of slots configured to form rectangles in a conductive layer. The rectangular cells of the first array may be registered with the rectangular cells of the second array, such that a plurality of the cells in the first array each have a corresponding cell in the second array. In addition, the unit cells of the first array may differ in their dimensions from the unit cells of the second array. According to still another embodiment of the present invention, the unit cells of the first array are registered with the unit cells of the second array such that the plurality of unit cells of the first array each have at least one edge that is not aligned with at least one edge of a corresponding unit cell of the second array. By changing the distance separating the first and second arrays of unit cells, the admittance of the lens can be controlled. This in turn allows the phase of radio frequency radiation propagating through the lens to be controlled.
According to an embodiment of the present invention, the distance between the first and second arrays is controlled by selectively introducing a voltage potential between the first and second arrays. In particular, by introducing a voltage differential between the first and second arrays, the surfaces of the arrays may be pulled closer to one another, thereby altering the admittance presented by the lens to an incident radio frequency wave. Upon removal of the voltage differential, an elastic force may return the distance between the arrays to a nominal distance. Such an elastic force may be provided by the deformation of at least a portion of a flexible substrate upon which at least one of the arrays is formed. Alternatively or in addition, the distance between the arrays may be restored to a nominal distance by introducing a potential difference between either the first array or the second array and a third surface.
In accordance with an embodiment of the present invention, a method is provided for steering a radio frequency electromagnetic wave. According to the method, a lens having reconfigurable frequency selective surfaces is positioned so that at least a portion of the electromagnetic wave that is to be modified is incident on the lens. The amount of phase shift imparted to the incident radiation is altered between at least first and second amounts by altering the distance between two frequency selective surfaces. In accordance with an embodiment of the present invention, this distance is altered by electro-mechanical means. In accordance with a further embodiment of the present invention, the distance between the two frequency selective surfaces is altered by introducing a voltage potential between the two frequency selective surfaces, or between one of the frequency selective surfaces and another surface.
In accordance with still another embodiment of the present invention, the unit cells of at least one of the frequency selective surfaces are divided into rows or columns such that the electrically conductive material surrounding a first of the rows or columns is electrically isolated from the electrically conductive material surrounding the adjacent rows or columns. According to such an embodiment, the phase shift imparted to incident electromagnetic radiation by one portion of the reconfigurable lens can be different from the phase shift imparted by other areas of the lens.
In accordance with still another embodiment of the present invention, a lens having a plurality of frequency selective surface pairs is provided. Within each pair, at least one of the frequency selective surfaces has columns or rows of unit cells that are electrically isolated from and movable in relation to adjacent columns or rows and that are moveable to the other frequency selective surface in the pair. According to such an embodiment, a plurality of phase shift amounts may be imparted by the reconfigurable lens to different portions of an incident electromagnetic wave. For example, the lens may be controlled to impart an ascending sequence of phase shift amounts across the width of the lens, to steer the incident electromagnetic radiation wave in a first dimension.
According to still another embodiment of the present invention, a plurality of frequency selective surfaces having columns of unit cells isolated from adjacent columns of unit cells are provided to steer an incident electromagnetic wave in a first dimension. In addition, a second plurality of frequency selective surfaces, having rows of unit cells electrically and mechanically isolated from adjacent rows of unit cells are provided to phase shift an incident electromagnetic wave in a second dimension. The frequency selective surfaces having their unit cells divided into columns are aligned with the frequency selective surfaces having their unit cells divided into rows such that the rows and columns are orthogonal to one another. The resulting reconfigurable lens assembly is capable of scanning radio frequency radiation incident on the lens in two dimensions.
According to yet another embodiment of the present invention, a reconfigurable radio frequency lens is provided by arranging a pair of frequency selective surfaces. Within each pair at least one of the frequency selective surfaces has rows or columns of unit cells that can be selectively moved so that a distance between the rows or columns of unit cells from the other surface can be altered to provide a selected phase shift amount. Furthermore, a plurality of pairs of frequency selective surfaces can be cascaded with one another to provide a lens capable of shifting incident radio frequency radiation by a plurality of phase shift amounts. If the cascaded FSS pairs both have columns (or rows) of unit cells that can be moved, all or a portion of an incident radio frequency wave can be steered in one dimension. If one of the FSS pairs has columns of unit cells that can be moved, and another of the FSS pairs has rows of unit cells that can be moved, an incident radio frequency wave can be steered in two dimensions.
According to a further embodiment of the present invention, a pair of FSS surfaces is capable of phase shifting at least a portion of incident radio frequency radiation by either of two amounts. Such a pair of FSS surfaces therefore forms a 1-bit lens. Multiple 1-bit lenses can be cascaded with one another to form a multiple bit lens.
According to still another embodiment of the present invention, a radio frequency lens or shutter may be provided by cascading surfaces or stages having resonant frequencies that can be altered or tuned. For example, surfaces with resonant frequencies that can be tuned using diodes or a tunable ferroelectric may be cascaded to provide a lens or shutter.
According to yet another embodiment of the present invention, a radio frequency shutter may be produced, wherein the amplitude of transmitted radio frequency waves through one or more pairs of FSS structures may be increased or decreased within a fixed frequency band. This is accomplished by de-tuning the FSS pair or pairs from a low loss resonant state to a higher loss non-resonant state.
Based on the foregoing summary, a number of salient features of the present invention are readily discerned. A reconfigurable radio frequency lens or shutter can be provided using pairs of frequency selective surfaces. Radio frequency radiation incident on the lens can be selectively phase shifted by altering the distance between the two frequency selective surfaces. Selected portions of the incident radio frequency wave can be phase shifted by controlling the distance separating individual rows or columns of unit cells of a first frequency selective surface from corresponding rows or columns of unit cells of a second frequency selective surface. The distance between the frequency selective surfaces of a pair of such surfaces can be controlled by applying a voltage potential between the two frequency selective surfaces or portions of those surfaces. By cascading multiple pairs of frequency selective surfaces together, a multiple bit reconfigurable radio frequency lens capable of pointing an incident beam of electromagnetic energy in space is provided. The reconfigurable lens features low insertion losses, and relatively simple construction and control techniques.
Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings.